1. Field of the invention
This invention relates to apparatus for producing high voltage pulses by capacitor discharge. One application of the invention is to provide apparatus for producing high voltage pulses by capacitor discharge for driving an X-ray tube in a transportable diagnostic X-ray unit.
2. Description of the prior art
Referring to the accompanying drawings, FIG. 1 is a circuit diagram of an X-ray tube "LOAD" connected to a conventional apparatus (not in accordance with the invention) for producing high voltage pulses by capacitor discharge, for driving the X-ray tube. More particularly, a very high-voltage transformer H.V. has a primary winding supplied with 240 volts AC at 50Hz, and has a very high-voltage secondary winding connected as shown via two diodes to two storage capacitors each of (say) 2.mu.F capacitance, to charge each capacitor slowly up to about 65kV. Since the two capacitors are in series and are charged by the circuit shown with opposite polarities, the total voltage across them is about 130kV. The capacitance, 2.mu.F, of each capacitor is such that the total energy then stored by the two capacitors is about 8 kilojoules. The capacitors are connected in series through two flashover protection resistances to the anode a and cathode k of the X-ray tube, which has a control grid connected to a control circuit for controlling the timing and duration of X-ray emission.
Unfortunately, because the control grid may not be 100% effective and because the 130kV is continuously across the anode and cathode, there may continuously occur some X-ray leakage from the tube, which is dangerous.
Furthermore, the desired quality of X-ray emission is such that the only useful pulse energy that is produced is that from a fall in voltage in the range from about 130kV to about 80kV or higher, so that only a small proportion of the stored energy is utilised. Hence the capacitors have to be much larger and heavier than they would if more of the energy could be utilised, which is disadvantageous in a transportable X-ray unit.
Furthermore, 8 kilojoules energy stored at 130kV in a transportable X-ray unit is potentially extremely dangerous, even fatally so, because of the possibility of a breakdown of electrical insulation .